In drug development, the stage of selection of drug candidate substances is of the highest interest to the pharmaceutical companies. It is common knowledge that enormous research and development cost of 30 billion yen or more and a period of 10 years or longer is required for the drug development. Therefore, it must be avoided that the drug candidate substance halts the development in the middle of the research and development. In preclinical studies, animals such as monkeys, dogs, rats and mice are used, however, it is known that a drug metabolic activity is greatly different between animals and humans. Because of the species difference of drug metabolic activity between humans and animals, development is halted in the stage of clinical studies with respect to about half the drug candidate substances.
In human hepatocyte chimeric mice, human pharmacokinetics can be reproduced in a state close to the case where evaluation is performed in human individuals. Therefore, it is considered that they can be used for the prediction of human pharmacokinetics in drug development. From these, human hepatocyte chimeric mice are considered to be effective as a tool for predicting metabolism or toxicity in humans in drug development. Further, if it becomes possible to produce chimeric mice carrying hepatocytes of a patient himself, it becomes possible to realize personalized medicine that can be used for considering dosing strategy or treatment method suitable for the pathologic conditions of the patient.
The present inventors succeeded in producing a human hepatocyte chimeric mouse in which 70% or more of the mouse liver was replaced with human hepatocytes by producing a uPA/SCID mouse which is obtained by mating a mouse (uPA-Tg mouse) into which urokinase plasminogen activator (uPA) gene ligated to an enhancer and a promoter of albumin was introduced with a SCID mouse, transplanting human hepatocytes into this mouse and administering a complement inhibitor (Patent document 1).
However, even in the method of Patent document 1, high replacement with human hepatocytes (70% or more) can be achieved in the case where young human hepatocytes (the age confirmed in Patent document 1 is 14 years or younger) are transplanted, and even if hepatocytes of an adult human who is 40 years old or older are transplanted into a uPA/SCID mouse, the replacement ratio is 5% or less in most of the cases.
There are some cases where the degree of drug metabolic activity is different between young human hepatocytes and adult human hepatocytes depending on, for example, the type of drug, therefore, the necessity of human hepatocyte chimeric mice in which the replacement with adult human hepatocytes is achieved at a high ratio is extremely high for drug metabolism or toxicity screening. Further, also in the case where chimeric mice carrying hepatocytes of a patient himself are produced for personalized medicine, adult human hepatocyte chimeric mice with high replacement are necessary for therapy for adult diseases.
Incidentally, it has been revealed that growth hormone promotes the proliferation of hepatocytes mediated by the expression of Foxm1B gene in hepatocytes, and it is known that by administering growth hormone to aged mice whose liver was excised, the growth potential of hepatocytes can be restored to the same level as that of young mice after the liver was excised (Non-patent document 1). In this connection, a method of treating a liver disease and liver damage using growth hormone and FoxM1B (Patent document 2), and a therapeutic agent for acute hepatic failure containing human growth hormone as an active ingredient (Patent document 3) are known, respectively. Further, IGF-1 (insulin-like growth factor-1) is also known as a hepatocyte growth factor, and it is also known that its expression is increased by growth hormone (for example, Non-patent documents 2 and 3).
On the other hand, as described above, in the liver of chimeric mice transplanted with young human hepatocytes, the replacement ratio with human hepatocytes reaches 70% or more on about day 60 after transplantation. These human hepatocytes on about day 60 after transplantation are morphologically similar to hepatocytes in the normal human liver, however, lipid droplets are gradually accumulated in hepatocytes thereafter in some cases. The chimeric mouse showing such fatty liver symptoms is useful as a fatty liver animal model, however, it is not suitable as a tool for studying the drug efficacy or toxicity for human liver. Accordingly, a means for improving fatty liver symptoms occurring in human hepatocyte chimeric mice with high replacement has been demanded.
Patent document 1: WO 03/080821 A1
Patent document 2: JP-T-2005-504010
Patent document 3: JP-A-9-136840
Non-patent document 1: K., Krupczak-Hollis, et al, Hepatology, 2003, 38:
Non-patent document 2: Velasco B., et al., Eur. J. Endocrinol. 2001, 145(1): 73-85
Non-patent document 3: Bartke A. et al., Proc. Soc. Exp. Biol. Med. 1999, 222(2): 113-23